Separation of chlorinated pyridines and derivatives thereof

ABSTRACT

The present disclosure is directed to a method for separating halogen substituted pyridines having an unhindered hydrogen atom in a position alpha or beta to the ring nitrogen from closely related halogen substituted pyridines which lack such a hydrogen atom. The separation is carried out by contacting a mixture of the above pyridines with sulfuric acid or an alkane sulfonic acid in the presence of a solvent for those species not possessing an unhindered hydrogen atom followed by a hydrolysis step. This method can also be employed to separate isometric polychloropyridines from each other.

United States Patent Kyriacou June 6, 1972 [54] SEPARATION OFCHLORINATED [56] References Cited PYRIDINES AND DERIVATIVES UNITEDSTATES PATENTS THEREOF 3,317, 42 5 1967 H ld' t l. ..26 72 Inventor;Demetrius Kyriacou, Oakley, Calif. 5 l me e 3 0/290 [73] Assignee: TheDow Chemical Company, Midland, Primary y Jiles Mich AssistantExaminer-Harry l. Moatz Attorney-Griswold & Burdick, S. Preston Jonesand C. Ken- [22] Filed: Apr. 8, 1971 th Bj rk [2]] Appl. No.: 132,55057] ABSTRACT Related Allplicafion Data The present disclosure isdirected to a method for separating [63] Continuatiomimpan of Ser.784,226 16, halogen substituted pyndines having an unhindered hydrogen1968 abandoned atom in a position alpha or beta to the ring nitrogenfrom closely related halogen substituted pyridines which lack such ahydrogen atom. The separation is carried out by contacting a [52] US.Cl. "260/290 A, 260/2949 v mixture of the above pyridines with sulfuricacid or an alkane [51 Ill. Cl. ..C07d 31/16 sulfonic acid in thepresence of a Solvent for those Species not [58] Field of Search..260/290, 294.9 possessing an unhindered hydrogen atom followed by ahydrolysis step. This method can also be employed to separate isometricpolychloropyridines from each other.

10 Claims, N0 Drawings inert SEPARATION OF CHLORINATED PYRIDINES ANDDERIVATIVES THEREOF CROSS-REFERENCE TO RELATED APPLICATION BACKGROUND OFTHE INVENTION This invention relates to the separation of closelyrelated halogen substituted pyridines from each other in a mixturecontaining these pyridines and is particularly concerned with theseparation and recovery of halogenated pyridines having an unhinderedhydrogen atom in a position alpha or beta to the ring nitrogen fromadmixture with other such substituted pyridines not possessing such ahydrogen atom.

Mixtures of halogen substituted pyridines, hereinafter calledhalopyridines, ordinarily result from liquid or vapor phase operationsconventionally employed for the halogenation of pyridine or substitutedpyridines, for example, picoline. To date, such mixtures have beenseparated only with difficulty using inefficient processes. Toillustrate, in US. Pat. No. 2,679,453 a methyl pyridine is chlorinatedwith gaseous chlorine in the presence of water at a temperature of from50-l50 C. resulting in the production of a mixture of halogensubstituted derivatives. This patent further teaches the recovery of theseparate products by fractional distillation under reduced pressure.

Other methods taught in US. Pat. Nos. 3,251,848 and 3,256,167 alsosuggest separation of the various halogenated pyridines by fractionaldistillation.

The use of fractional distillation to separate mixtures of halogenatedpyridines into their components has a number of disadvantages. One suchdisadvantage is the large number of plates required in the distillationcolumn for efficient separation which requires a high monetary outlay inequipment.

The present invention avoids these'difficulties and makes for a cleanseparation of halogenated pyridine components present in mixtures alongwith high levels of product recovery. The instant process also providesan economical method for separating polyhalogenated pyridines from eachother which can be practiced using conventional in-plant equipment.

SUMMARY OF THE INVENTION The present invention relates to a method forseparating halogen substituted pyridines having an unhindered hydrogenatom in a position alpha or beta to the ring nitrogen from closelyrelated hydrogen substituted pyridines which lack such a hydrogen atom.

This inventionalso contemplates separations of various halopyridinessuch as those wherein all of the halogen substituents can be the same,or wherein the compounds have mixed halogen moieties, i.e., thosecompounds having chlorine, bromine, fluorine or iodine or mixed halogensubstituents. Also contemplated are separations wherein groups otherthan halogen can be attached directly to the pyridine ring such ascyano, nitro, or loweralkyl groups of one to four carbon atoms.

One specific advantage of this invention is that it permits separationsbetween halopyridine isomers which heretofore have been very difficult,or almost impossible, to achieve; and separations betweenpolyhalopyridines and monohalopyridines and/or unhalogenated pyridinesby a simple, rapid and inexpensive method.

In general, the method of the present invention comprises intimatelycontacting in any convenient fashion a polyhalopyridine containingmixture having pyridine species both with and without unhinderedhydrogen atoms alpha or beta to the ring nitrogen, with sulfuric acid,an alkane monosulfonic acid or alkane disulfonic acid, preferably, anliquid carrier, which is a solvent for those polyhalogenated pyridinespecies in the mixture which do not possess an unhindered alpha or betahydrogen atom is also employed.

kane group containing from one to about five carbon atoms.

Representative sulfonic acids include, for example, methanesulfonicacid, methanedisulfonic acid, ethanedisulfonic acid, 2-propanesulfonicacid, butanesulfonic acid, 2- methypropanel-sulfonic acid,S-methylbutanesulfonci acid, ethanedisulfonic acid, B-pentanesulfonicacid and pentanedisulfonic "cid, among others.

Ordinarily, in the practice of this invention, a product stream from aliquid or vapor phase pyridine halogenation process containing a mixtureof halogenated pyridines is neutralized and intimately contacted withconcentrated sulfuric acid or an alkane (monoor disulfonic acid, eitherbefore or after the product stream is mixed with a carrier liquid whichis immiscible with the acid and which is a solvent for thosepolyhalogenated pyridine species which lack an unhindered alpha or betahydrogen atom, and which is of a specific gravity such as to allow readyseparation from the acid. It is preferable, from the standpoint of easeof operation and convenience, that the liquid carrier be mixed with thehalogenated pyridines prior to contact with the acid.

After a thorough mixing of the acid with the halogenated pyridines, themixture is allowed to separate into two phases, a lower acid phase whichcontains the acid reaction product of the unhindered hydrogen atomcontaining pyridines and an upper liquid carrier solvent phasecontaining the halopyridines lacking such a hydrogen solvent atom. Thesolvent phase is separated by decantation or other conventionalseparatory procedure, washed with dilute aqueous base to insure acidremoval and stripped of solvent to recover the pyridines containedtherein. Any tendency towards emulsification during the water washingstep can be overcome by addition to the wash water of 1 gram of sodiumchloride per 100 milliliters of water. The acid phase is diluted withsufficient water to decompose the acid pyridine reaction products. Thehalopyridines are insoluble in the diluted acid and can be easilyrecovered by dissolution in a solvent therefor, such as one of thesolvents mentioned hereinbefore, followed by solvent stripping or bysome other conventional method to remove the solvent leaving thehalopyridine as a residue.

It is feasible at the initial stage of operation to carry out thecontacting of the polyhalopyridine containing mixture with the acid inthe absence of a solvent to convert at least a portion of thosepyridines not having an unhindered hydrogen atom and which have notundergone reaction with the acid. However, from a practical standpoint,this is not very workable even on those materials which are solid. Theseseparations are slow and/or incomplete and particularly difficult withthose materials which are liquid. Additionally, premixing a liquidcarrier with the halopyridine mixture before the acid addition offersthe advantage of serving to control the rate of heat evolution duringacid addition. Also, local overheating is prevented or greatly minimizedand more intimate contact between the various components of the systemoccurs.

If sulfuric acid is employed, this reactant should have a concentrationof at least 70 percent H 50 and preferably, it should be of from aboutto about 98 percent H SO The amount of either sulfuric acid or alkane(monoor disulfonic acid employed can vary and is usually present insufficient quantity to provide a molar ratio of at least 1 mole of acidper mole of halogenated pyridine. It is preferable for ease of operationand from a standpoint of economy to have the acid present in the ratioof from 2 to 7 moles of acid per mole of halogenated pyridine since thehigher yields are obtained with larger amounts of acid.

The process can be carried out over a wide temperature range rangingfrom 0 C. up to the boiling point of the solvent or the temperature atwhich the acid attacks the halogenated pyridines or solvent, whicheveris the lower. The preferred temperature range is between about 25 and 65C. The contact time is not critical as long as it is sufficient tocomplete the reaction. A minimum contact time between the acid and thehalogenated pyridines of at least about 0.001 second appears to besufficient for the reaction to proceed. However, in order to insureintimate and thorough contact between the acid and the halopyridines, itis preferred that a contact time of between about 1 and about 30 minutesbe employed. Longer contact times of up to 2 or 3 days do not adverselyaffect the results provided the temperature is maintained low enough toprevent decomposition of the halogenated pyridines or solvent by theacid.

The inert liquid carrier can be any material which is (1) liquid at thereaction conditions, (2) nonreactive and immiscible with the acid and(3) which is preferentially a solvent for those halopyridines notpossessing an unhindered hydrogen atom in a position alpha or beta tothe ring nitrogen.

Representative inert liquid carriers include dichloromethane, hexane,pentane, heptane, octane, isopentane and carbon tetrachloride. Thespecific liquid carrier to be employed in a given operation will dependon the results desired from a predetermined mixture and readily can bedetermined by one skilled in the art. For most operations, the preferredliquid carriers are dichloromethane and hexane. The liquid carrier isusually employed in an amount of between about 2 and 10 volumes ofcarrier per volume of acid.

DESCRIPTION OF SOME PREFERRED EMBODIMENTS The following examplesillustrate the best methods now known for the practice of the presentinvention and will enable those skilled in the art to practice theinvention.

EXAMPLE 1 A qualitative indication of whether or not a mixture ofpolyhalopyridines contains species separable by the practice of thepresent invention can be obtained without a detailed knowledge of thestructures of the several species present by making the following test.

TEST PROCEDURE Equal volumes of a polyhalopyridine and concentratedsulfuric acid are mixed by slowly pouring the acid into the mixture withstirring and observing the temperature rise if any. A temperature riseof 5 C. or less indicates that no appreciable proportion of salt formingspecies are present.

ln a representative operation, a number of polyhalopyridine compoundswith and without an unhindered hydrogen atom in a position alpha or betato the ring nitrogen are separately screened to determine their abilityto form salts with sulfuric acid. To separate 5O milliliter round bottomflasks is added a 3 gram portion of one of the test compounds. The flaskis fitted with a thermometer and agitates while each test compound ismixed with 4 grams of concentrated sulfuric acid (about 96 percent H 80The resultant temperature rise is recorded. To each mixture is added 25milliliters of dichloromethane with agitation and each allowed to standwhereupon the mixture phase separates into an acid layer and adichloromethane layer. In each, the dichloromethane layer is separatedby decantation, the dichloromethane removed by stripping and the residueremaining weighed. The acid layer is diluted with milliliters of waterand any precipitate formed is taken up with dichloromethane. Thedichloromethane is removed by stripping and the residue weighed. Theresults of this screening is given in the following table:

and 2,3,5,6-tetrachloropyridine mixture (3 grams each) and equal volumesulfuric acid 'Dichloromethane "Mixture of 2,4 5,6- and3,4,5,6-tetrachlorocyanopyridine isomers EXAMPLE 2 To an agitatedmixture containing grams of a mixture of 2,3,4,5-tetrachloropyridine (88weight percent), 2,3,5,6- tetrachloropyridine (8 weight percent) andpentachloropyridine (4 weight percent) dissolved in milliliters ofdichloromethane is added 60 milliliters of about 96 percent sulfuricacid. A temperature increase of 40 C. is observed with the formation oftwo phases. Agitation is continued for a few additional minutes afterwhich the phases are allowed to separate. The upper dichloromethanelayer is decanted and the lower acid layer is washed three times with100 milliliter portions of fresh dichloromethane. The dichloromethanedecants are combined with the other dichloromethane layer and this totalmixture is washed with water containing a few drops of concentratedammonium hydroxide. The dichloromethane is removed by stripping, leavinga pot residue of l 1.5 grams of a white crystalline solid which whenanalyzed by vapor phase chromatography is found to consist of 60 molepercent 2,3,5,6- tetrachloropyridine, 35 mole percentpentachloropyridine and 5 mole percent 2,3,4,S-tetrachloropyridine. Theacid layer is added with agitation to 500 milliliters of cold water andthe chloropyridine content separates as a second liquid phase. About 50milliliters of dichloromethane is added with agitation and the mixtureallowed to phase separate. The aqueous acid-containing upper layer isdiscarded and the lower dichloromethane-containing layer washed withwater followed by a wash with water containing a few drops of ammonia.The dichloromethane is removed by stripping leaving a pot residue of 88grams of a liquid residue (white solid at about 15 C.) which is analyzedby vapor phase chromatography and infrared spectroscopy and is found toconsist of 99.5 percent pure 2,3,4,5-tetrachloropyridine.

EXAMPLE 3 A polychloropyridine separation is carried out in a mannersubstantially as described in Example 2, except that starting mixtureconsists of 200 grams of a mixed feed containing3,4,5,6-tetrachloro-2-(trichloromethyl)pyridine (82 weight percent),3,4,5-trichloro-2-(trichloromethyl)pyridine 16weight percent) and3,5-dichloro-2- (trichloromethyl)pyridine (2weight percent) dissolved in100 milliliters of dichloromethane. While this mixture is beingagitated, 112 grams of 86 percent sulfuric acid is slowly added. Afterphase formation and separation, 163.2 grams of 3,4,5,6-tetrachloro-2-(trichloromethyl)pyridine having a purity of 98 percent is recovered fromthe upper dichloromethane layer and 31.2 grams of a mixture of3,4,5-trichloro-2- (trichloromethyl )-pyridine and 3 ,5-dichloro-2-(trichloromethyl)pyridine is recovered from the acid layer. A mechanicalloss of 5.6 grams is incurred.

In another run following the above procedure wherein 96 percent sulfuricacid is used, the purity of the 3,4,5,6-tetrachloro-2-(trichloromethyl)pyridine recovered is 99.5 percent.

EXAMRLE, 4

A sample of the crude mixed product from a liquid phase chlorination ofgamma picoline (as the dihydrochloride) is resolved by vapor phasechromatography into 9 peaks (fractions).

Peaks 1 through 8 constitute 62.4 mole percent of the components of theeluents and peak 9, which was trapped on exiting from the vapor phasechromatographic column, constituted 37.6 percent of the eluents and uponmass spectrographic analysis is found to consist primarily of 2,3,5,6-tetrach1oro-4-(trichloromethyl)pyridine with a small amount of2,3,6-trichloro-4-(trichloromethyl)pyridine. The inability of the latterspecies, i.e., which has a hydrogen in the beta position to the nitrogenbut also has an adjacent trichloromethyl group to form a salt withsulfuric acid, is thus demonstrated.

EXAMPLE 5 A 150 gram sample of the crude polyhalopyridine productmixture from the same source as employed in Example 4 is subjected to anaspiration step to remove any free chlorine present and mixed with 200milliliters of dichloromethane. The mixture is agitated and to thisagitated mixture is added 150 grams of technical grade concentratedsulfuric acid. Agitation is continued for a few minutes, after which themixture is allowed to separate into distinct phases. The less densedichloromethane layer is decanted. Two additional 150 gram samples ofthe crude product are mixed with 200 milliliters of dichloromethane andtreated in the same manner with 200 gram and 173 gram samples of freshacid, respectively. The resulting dichloromethane layers which separateare recovered, combined and mixed with 139 grams of fresh acid. Afterseparation from this last stage, the acid layers are combined andextracted with three 500 milliliter portions of fresh dichloromethane.This latter portion, after separation from the acid layer, is combinedwith the other dichloromethane extracts. The extract mixtures arecontacted with 150 grams of fresh acid, separated and washed with wateruntil the wash water becomes neutral to pH paper. The dichloromethanesolvent is removed by stripping, leaving a pot residue which is found tobe 157.5 grams of 91 percent pure 2,3,5,6-tetrachloro-2-(trichloromethyl)pyridine.

EXAMPLE6 A mixture containing 1.8 grams of 2,3,4,5- tetrachloropyridineand 0.2 grams of 2,3,5,6-

tetrachloropyridine is mixed in an open reactor with 25 milliliters ofhexane and 4 grams of ethanesulfonic acid. A temperature rise of 20 C.is measured. The mixture is allowed to phase separate. A quantitiveyield of 2,3,4,5- tetrachloropyridine is recovered from the acid layerand a quantitative yield of 2,3,5,6-tetrachloropyridine is recoveredfrom the hexane layer.

Additional runs are made in the same manner with fresh portions of thesame polyhalogenated pyridine mixture employing etlranedisulfonic acid,methanesulfonic acid and methanedisulfonic acid. In each run, separationof 2,3,4,5- tetrachloropyridine and 2,3,5,6-tetrachloropyridine isrealized.

EXAMPLE 7 In an additional experiment, separate 10 gram samples of (a)2,3,4,S-tetrachloropyridine, (b) 2,5-dichloro-pyridine and (c) 3,4,5trichloro-2 (trichloromethyl)pyridine are each mixed with 2 grams of 98percent sulfuric acid.-Each mixture is separately extracted with a totalof 100 milliliters of dichloromethane. After extraction, there remains avery viscous semiliquid. Each of the liquids is weighed in a dry box andanalyzed by base titration to determine the sulfuric acid content. Thesecontents as set forth below indicate the formation of the correspondingsulfuric acid salt for each of the pyridines. 1

Pyridine/Acid Acid Mole Ratio of Content Compound Composition by Weight2,3 ,4,5tetrachloropyridine 1 :2 46% 2,5-dichloropyridine 1 :2 56%3,4,5-trichloro- 2-(tri- 1:2 46% chloromethyl)pyridine EXAMPLE 8 MolarRatio of 86% Acid to Pyridine Compounds Percent Yield of2,3,4,5-Tetrachl0ropyridine In a similar operation using the samepyridine mixture and 96 percent sulfuric acid, it is found that a ratioof 2 to 3 moles of acid per mole of pyridine compound suffices toprovide at least about 80 percent separation and recovery of 2,3,4,5-tetrachloropyridine.

In another similar operation, it is found that similar recoveries of3,4,5-trichloro-2-(trichloromethyl)-pyridine using 86 percent sulfuricacid are realized at acid/pyridine compound mole ratios of from about 5to about 7.

EXAMPLE 9 A series of runs are made employing portions of thechloropyridine mixture as in Example 2. In each of these runs, 100 gramsof the chloropyridine mixture is mixed with 200 milliliters ofdichloromethane and to this mixture is added grams of sulfuric acid. Ineach run, the acid concentration is varied. After acid addition, themixture is allowed to phase separate. 500 Milliliters of water is addedto the acid layer to reconvert the acid salt to the original pyridinewhich is extracted with fresh dichloromethane. The solvent is strippedfrom the organic extract and the 2,3,4,5-tetrachloropyridine content ofthe residue determined. The results of these runs as set forth in thetable below show the relationship of the acid concentration of theseparation or yield of 2,3,4,5- tetrachloropyridine.

ased on total 2,3,4,5-tetrachloropyridine in starting mixture Thealkanesulfonic acids employed herein either can be obtained fromcommercial suppliers or can be prepared by following known procedures tooxidize the appropriate lead mercaptide to the corresponding leadsulfonate with nitric acid followed by a conversion to the free acidwith dry hydrogen chloride in isopropyl alcohol. The alkane disulfonicacids are also available as commercial items or can be prepared by theknown reaction of an appropriate bis-alkyl halide with a concentratedaqueous solution of sodium, potassium or ammonium sulfate, followed by aconversion of the salt to the free acid. These preparations are furtherdescribed in Synthetic Organic Chemistry", Wagner et al, (1953) pages811-820, John Wiley & Sons, Inc. New York.

WHAT IS CLAIMED l8:

1. A method for separating halogen substituted pyridine speciescontaining at least one unhindered hydrogen atom in a position alpha orbeta to the ring nitrogen and corresponding to the formula wherein eachof R R R R and R independently represents hydrogen, chloro, fluoro,bromo, iodo, nitro, cyano, trichloromethyl and loweralkyl of one to fourcarbon atoms, with the proviso that at least one of R R R and R must behydrogen and that at least one of R R R R and R must be chloro, fluoro,bromo or iodo, from admixture with closely related halogen substitutedpyridine species not having an unhindered hydrogen in the said alpha orbeta position and corresponding to the formula wherein each of R R R'and R independently represents chloro, fluoro, bromo, iodo, nitro,cyano, trichloromethyl and loweralkyl of one to four carbon atoms and Ris as hereinabove set forth, with the proviso that at least one of R R RR and R must be chloro, fluoro, bromo or iodo, which comprises,contacting a mixture of the hereinabove set forth halogen substitutedpyridine containing species having at least one unhindered hydrogen atomin a position alpha or beta to the ring nitrogen and species not havingan unhindered hydrogen in said alpha or beta position, with at least oneof concentrated sulfuric acid having an acid strength of from 70 to 98percent H 80 an alkane sulfonic acid containing from I to about 5 carbonatoms or an alkene disulfonic acid containing from one to about fivecarbon atoms, said acid being present in a molar ratio of from 1 to 7moles per mole of said halogen substituted pyridines in the presence ofan inert liquid carrier, said carrier being a solvent for the halogensubstituted pyridines not having an unhindered alpha or beta hydrogenatom and being essentially immiscible with said acid, said contacting iscarried out at a temperature of from 0 C. to the temperature at whichthe acid attacks the halogenated pyridines for a period of timesufficient to complete the reaction and thereafter separating thespecies not having an unhindered hydrogen atom therefrom.

2. The method according to claim 1 wherein the halogen substitutedpyridine containing mixture is mixed with said inert liquid carrierprior to said acid contacting.

3. The method according to claim 1 wherein the halogen substitutedpyridine containing mixture is a product from the halogenation of apyridine.

4. The method according to claim 3 wherein the mixture contains2,3,4,5-tetrachloropyridine, 2,3,5,6- tetrachloropyridine andpentachloropyridine.

5. The method according to claim 3 wherein the mixture contains3,4,5,6-tetrachloro-2-(trichloromethyl)-pyridine,3,4,5-trichloro-2-(trichloromethyl)pyridine and 3,5-dichloro-2-(trichlorometh l)pyridine.

6. The metho according to claim 3 wherein the mixture contains2,3,5,6-tetrachloro-4-(trichloromethyl)-pyridine and 2,3,6-trichloro-47.The method according to claim 2 wherein Y the acid is concentratedsulfuric acid, the liquid carrier is dichloromethane and the halogensubstituted pyridine recovered is 2,3,4,S-tetrachloropyridine.

8. The method according to claim 5 wherein the acid has a strength of atleast 70 percent up to 98 percent H 50 said acid being present in amolar ratio of from 1 to 7 moles per mole of said halogen substitutedpyridines and the liquid carrier is present in the amount of from about2 to 10 volumes per volume of acid.

9. The method according to claim 6 wherein the acid has a strength ofbetween about to about 96 percent H 50 said acid being in a molar ratioof from 2 to 7 moles per mole of said halogen substituted pyridines andthe liquid carrier is present in the amount of from about 2 to 10volumes per volume of acid.

10. The method according to claim 2 wherein the acid is one of an alkanesulfonic acid or an alkane disulfonic acid, the liquid carrier is hexaneand the halogen substituted pyridine recovered is2,3,4,5-tetrachloropyridine.

2. The method according to claim 1 wherein the halogen substitutedpyridine containing mixture is mixed with said inert liquid carrierprior to said acid contacting.
 3. The method according to claim 1wherein the halogen substituted pyridine containing mixture is a productfrom the halogenation of a pyridine.
 4. The method according to claim 3wherein the mixture contains 2,3,4,5-tetrachloropyridine,2,3,5,6-tetrachloropyridine and pentachloropyridine.
 5. The methodaccording to claim 3 wherein the mixture contains3,4,5,6-tetrachloro-2-(trichloromethyl)-pyridine,3,4,5-trichloro-2-(trichloromethyl)pyridine and3,5-dichloro-2-(trichloromethyl)pyridine.
 6. The method according toclaim 3 wherein the mixture contains2,3,5,6-tetrachloro-4-(trichloromethyl)-pyridine and2,3,6-trichloro-4-(trichloromethyl)pyridine.
 7. The method according toclaim 2 wherein the acid is concentrated sulfuric acid, the liquidcarrier is dichloromethane and the halogen substituted pyridinerecovered is 2,3,4,5-tetrachloropyridine.
 8. The method according toclaim 5 wherein the acid has a strength of at least 70 percent up to 98percent H2SO4, said acid being present in a molar ratio of from 1 to 7moles per mole of said halogen substituted pyridines and the liquidcarrier is present in the amount of from about 2 to 10 volumes pervolume of acid.
 9. The method according to claim 6 wherein the acid hasa strength of between about 80 to about 96 percent H2SO4, said acidbeing in a molar ratio of from 2 to 7 moles per mole of said halogensubstituted pyridines and the liquid carrier is present in the amount offrom about 2 to 10 volumes per volume of acid.
 10. The method accordingto claim 2 wherein the acid is one of an alkane sulfonic acid or analkane disulfonic acid, the liquid carrier is hexane and the halogensubstituted pyridine recovered is 2,3,4,5-tetrachloropyridine.